"Metal fiber concrete" as used herein, is intended to mean a body of cementitious matrix including metal fibers and obtained by setting of a cementitious composition mixed with water.
"Concrete element" as used herein, is intended to mean columns, beams, slabs, walls, panels, protective panels, cladding panels and any decorative or structural element used in a construction.
Conventional concrete has a granular structure made up of the following three phases:
cement which constitutes the binding phase and has a grain size lying in the range 1 micrometer to 100 micrometers; PA1 sand which has a grain size lying in the range 1 mm to 4 mm; and PA1 coarse aggregate having a size lying in the range 5 mm to 20 mm, or 5 mm to 25 mm. PA1 at about 60.degree.-100.degree. C. during about 6 hours to 4 days starting after the end of the setting; PA1 at about 60.degree.-100.degree. C. during about 12 hours to 24 hours starting after the end of the setting; PA1 at about 60.degree.-100.degree. C. during about 6 hours to 4 days, starting at least one day after the beginning of the setting; PA1 at about 70.degree.-90.degree. C. during about 6 hours to 4 days after the end of the setting.
Conventional metal fiber concretes include steel fibers of length lying in the range 30 mm to 60 mm. The maximum length of fiber that can be used is limited firstly by the need to be able to perform mixing without excessive damage, and secondly by the casting requirements for the concrete (putting into place and vibration).
Smooth metal fibers are held in place in the concrete by adhesion. To ensure that a smooth fiber behaves well, it is important that the form factor, that is the length of the fiber divided by its diameter, lies in the range 50 to 100. This optimum form factor may be smaller if fiber anchoring is improved by a change in fiber shape: corrugations, end hooks, undulation, etc. . . . .
The concentrations of fibers in conventional metal fiber concretes lie in the range 30 kg/m.sup.3 to 150 kg/m.sup.3. They generally lie in the range 40 kg/m.sup.3 to 80 kg/m.sup.3, which corresponds to a volume percentage lying in the range 0.5% to 1%.
Fiber length L generally lies in the range 30 mm to 60 mm, whereas the diameter D of the coarsest aggregates generally lies in the range 20 mm to 25 mm, such that the ratio R=L/D lies in the range 1.2 to 3.0.
In conventional concrete, the interface between aggregate and set cement constitutes a zone of weakness because of its greater porosity (transition area). This interface is also the seat of local stresses due to the anisotropic behavior between the aggregate and the cement. When overall traction is applied to the concrete, the aggregates can no longer remain tied together unless there exist fasteners that withstand the traction and that extend over a length of not less than about ten times the size of the coarsest aggregate.
Since the ratio R is no more than 3.0, fibers are not effective in tying individual aggregates together.
That is confirmed by the fact that adding metal fibers to traditional concrete provides little improvement to the tensile strength (flexural strength) of the concrete. The improvement is of the order of a few percents for the usual fiber concentrations of 0.5% to 1% by volume.
The metal fibers used in concretes that do not include conventional reinforcement do not enable cracking of the concrete to be avoided, it only improves crack distribution, i.e. a large number of microcracks are obtained that are "sewn-together" by the fibers, instead of obtaining a smaller number of cracks that are larger.
Consequently, the use of conventional metal fiber concretes without conventional passive reinforcement is limited.
Particular cementitious compositions and processes are known for obtaining cementitious matrix comprising metal fibers (COMPRESIT, SIFCON, and others) and are disclosed f.i. in the U.S. Pat. Nos. 4,979,992 to H. H. BACHE, 4,513,040, 4,559,881, 4,593,627 and 4,668,548 to D. R. LANKARD.